1. Field of the Invention
This invention relates to the measurement of fluids flowing in pipes or conduits by use of the doppler sonic flow measurement technique. In such measurements, sound waves are transmitted through the walls of a pipe, and are received from the pipe again through the walls. A reliable coupling between the sonic transducers and the pipe walls must be provided for efficient transmission and reception of sound. The present invention provides an improved liquid coupling for sound transmission and reception to and from the walls of a pipe. This coupling provides temperature compensation and the capability of bubble elimination.
2. Description of the Prior Art
In ultrasonic flowmeters, it has been common to attach the transducers to the walls of a pipe by means of epoxy resins, grease, and liquid couplings which utilize coupling wedges of different materials.
Doppler sonic flowmeters operate by directing a transmitted beam of sound energy into a pipe within which the fluid flow is being measured. The sonic energy in doppler systems is scattered from the macroscopic or microscopic bubbles, particles and/or eddy currents in the moving fluid. A sonic receiver detects the scattered energy and provides an electrical signal representative of the scattered energy to an electronic detection unit which in turn calculates the flow. The calculation of the flow is based upon the doppler frequency shift between the transmitted sonic wave and the received sonic wave. In some applications, the same sonic transducer is used for both transmitting and receiving, while in other systems separate transducers are used for transmission and reception.
It is essential in such doppler systems that the sonic transmit and/or receiver sensor be placed at an angle with respect to the direction of fluid flow. Placement of the sensors directly perpendicular to the direction of flow does not provide a meaningful measurement. Typically, a wedge of material is placed between the transducer and the pipe carrying the fluid whose velocity is being measured to provide the desired angle between the sensor and the fluid flow.
In such systems, the flow indication is inversely proportional to the velocity of sound through the wedge material. Consequently, it is desirable that the wedge material have a low sound transmission-temperature coefficient to ensure the desired flow indication accuracy over a given temperature range. In systems which employ electronic compensation of the temperature effects, it is necessary that the sound transmission-temperature coefficient of the wedge material be accurately known.
Conventional systems utilizing wedges have employed hard materials, such as steel, aluminum and glass. These materials have fairly low sound transmission-temperature coefficients. However, such materials also exhibit a very high velocity of sound propagation, whereas the material between the wedge and the pipe, such as grease and epoxy adhesives, typically exhibit much lower velocity of sound propagation values. Consequently, there is a very high degree of refraction and a very large acoustic impedance mismatch between this type of wedge and such interface materials causing most of the signal to be lost due to reflection.
In systems which employ wedges fabricated from plastic or polyester materials, sound transmission temperature coefficients in the order of 0.1 percent per degree centigrade to 0.2 percent per degree centigrade are common. Such coefficients require electronic temperature compensation for accurate fluid flow measurements.
As used in this specification, the term sound transmission temperature coefficient is defined as the change of sound transmission velocity with respect to temperature, typically expressed as a percentage per degree centrigrade.
Another problem associated with conventional solid wedge materials is that of physically coupling the said wedges to the pipes which carry the fluid flow being measured. Conventionally, coupling has been achieved by the use of epoxy adhesives or coupling greases. Both epoxy adhesives and coupling greases, however, cause serious technical problems due to the presence of air bubbles or gas trapped in such materials. Such air bubbles, even at microscopic levels, detrimentally effect the accuracy of the doppler sound measurement. A high level of entrapped air bubbles present in the coupling can even cause a complete loss of measurement capability of the sonic doppler flowmeter. Further, it is not always possible to determine whether the flowmeter is operating at its full accuracy because of the uncertainty of the bubble formation in the coupling.
In the prior art use of solid wedge materials in combination with epoxy or grease couplings, there has developed yet another problem associated with the use of such flowmeters in the field. In field use, it is not always possible to achieve uniform mixtures of the epoxy or grease materials, or to achieve the desired bubble free condition of such materials. In the case of epoxy resins, the mere aging of the resins typically results in a change in the sound transmission characteristics of such resins, which change may require further calibration to insure the desired measurement accuracy.
The use of solid wedges within fluid couplings to compensate for temperature effects on the velocity of sound propagation is known in the art. For example, U.S. Pat. No. 3,913,386 employes a liquid coupling which forms the wedge between the ultrasonic flowmeter transducer element and the conduit through which the flow is being measured. As shown therein, a wedge of material having a sound transmission temperature coefficient which is opposed to that of the liquid is placed in the path of propagation of the sonic energy. In this technique, the opposite sound transmission temperature coefficients of the two different materials are used to compensate for the sound transmission temperature coefficient which otherwise is present in the fluid alone. In this system, however, substantial attenuation of the sonic energy is experienced because each interface between the fluid and the solid wedge material produces a substantial attenuation loss.
Furthermore, the system disclosed in U.S. Pat. No. 3,913,386 is utilized for ultrasonic testing of materials, such as metallic welds. The disclosure relates only to a method of temperature compensation for the angle of refraction of the sonic beam by use of two materials, one solid and one liquid, between the emitter of the sonic beam and the material to be measured. Thus, this disclosure relates to a compensation method which utilizes two wedges of different materials and physical states, and does not employ a single wedge of uniform material with a controllable constant temperature coefficient. This disclosed system is not analogous to doppler measurement techniques. Rather, it relates to a measurement which relies upon refraction and, thus, its purpose and application are entirely different from the apparatus and method of the present invention.